The present invention relates to switching mode power supplies, and in particular, to switching mode power supply controllers utilizing pulse width modulation to control the on times of the power transistor output stages. In such switching mode PWM controllers, the designs are using higher switching frequencies to take advantage of the reduced inductance/magnetic sizes that can be used at these frequencies. A major disadvantage or operating at higher switching frequencies is the increased switching and gate drive losses, resulting in lower system efficiency and increased board heat, that is, a rise in temperature. In certain applications, this increase in temperature cannot be tolerated due to reliability and system integrity concerns.
In the prior art, it is known that some PWM control ICs have a feature whereby the controller enters a hysteretic or “light load” mode. The controller enters this mode under low output current conditions, and the regulator only delivers power to maintain the output voltage. Consequently, the switching frequency can vary. This method is intended to reduce power loss to extend battery life in battery-powered applications. However, this prior art circuit does not provide for reduction in switching frequency as the temperature of the circuit increases. Further, it applies to a hysteretic mode converter whereby the power transistor turns on only to maintain the output voltage. Given this control, the frequency can vary depending on the output load. According to the present invention, it is desired to provide a controller which operates in a pulse width modulation mode whereby the switching frequency is maintained in a constant frequency operation but will change as a function of temperature changes. Accordingly, in contrast to the hysteretic converters wherein the switching frequency constantly changes as a function of output load, the present invention changes the switching frequency based on temperature change during steady state PWM operation.